With a schematic, it's always possible to fab a wire harness from scratch; that's a worst-case. In between taking that drastic step and sourcing NOS, there is the possibility of restoring/repairing the existing harness. If the leads are all still there, they can be used as templates. The biggest hassle would be sourcing wire with the desired color-coding. Beyond that, it's an afternoon of tedious cutting/stripping/soldering/bundling/wrapping...but...you only have to do this once.
Repairing a rusted-out tank is possible, but labor-intensive. Since the rot is normally on the bottom of the tank, the surgery doesn't have to be plastic surgery, if you take my meaning. Typically, the center section of the tank bottom will be fine, the damage concentrated in localized spots, at the outer perimeter, with some extending upward at the sides... usually less than 1.5", or less, above the lower edge. The best repair method is somewhat subjective. There's more than one "right answer to this multiple choice test". First step is rust removal, to see what is actually solid metal. With a tank that's more like a coarse sieve, chemical rust removal requires a plastic container bigger than the tank, plus a lot of solution. Evaporust is a good product but, at $27/gallon it ain't cheap...and...it has its limitations. It absolutely will not eliminate heavy rust. It also should be followed by a final treatment with phosphoric acid.
Since you're already thinking in terms of tank replacement, what have you got to lose by taking a few steps toward evaluating the original tanks? First thing I'd do is glass bead blast the rusted-through areas from the outside of the tank...and as much as the gun can reach, via the filler neck. That'll give you a much better idea of how much solid metal remains. It will also remove a lot of the heavier scaling, which will allows the chemical baths to work faster and more effectively. Small holes can be patched. With a clean, "white metal" surface, you've the option of welding, brazing or using a POR15 tank sealer kit...which includes fiberglass cloth, specifically for this purpose. If there are big sections of rotted metal, they can be cut out and new metal welded/brazed/sealed. Before the new metal goes in/on, the gaping holes can be used for bead blasting access...for the rest of the tank. After that, welding + brazing should seal up the tank well enough to contain the chemical solutions long enough that you won't have to immerse the entire tank.
I've restored a handful of Z50 tanks as described, they're similar to what you have. The cutting & templating is easy enough, since the tank bottom is flat (in the affected areas). The welding isn't fun; it's very difficult to get perfect sealing, unless one is an expert at TIG welding...which I am not. That's where brazing can be very effective. The welding is for strength, brazing seals most, if not all, of the pinholes. Regardless, the tank must be sealed afterward and that also takes care of any roughness along the weld seams. Based on this, I can tell you that this is tedious and it's not cheap. Between the brazing, Evaporust, phosphoric acid and POR15 products, you'll be into this for at a buck-and-a-half...doing a pair of these tanks at once, it won't cost much more. The rust dissolvers can be used twice and there's more than enough sealer in one kit, to do two tanks. If you have to farm-out the welding/brazing, I could see 150-200 per tank additional. Media blasting a tank should be cheap, $20, tops.
Now consider the price of an NOS tank, or a pair of them...$500/$1000...vs your cost, and effort, of restoring the originals, which could range anywhere from $150/$300 up to $350/$700, exclusive of your time. What will the rusty originals bring on the open market? If you can get $200 each from them, NOS replacements seem like a fair value and maybe a much better way to go. You're the only person who can make the decision. I'm trying to provide information that will allow you to make an informed decision.
